I. Field of the Invention
This invention relates generally to infrared line scanner (IRLS) systems, forward-looking infrared (FLIR) imaging systems, infrared search/track systems, laser printers and other electro-optical systems employing detector-array and/or emitter-array scanning with the requirement for fast, precise and accurate control of resolution and/or field coverage with electronic zoom.
II. Discussion of the Prior Art
Referring to FIG. 1, infrared line scanners (IRLSs) installed in reconnaissance aircraft typically use a linear detector array, i.e. a detector array consisting of a line of uniformly-spaced, equal-sized detector elements. The linear array is scanned over a constant-width swath on the ground at a constant angular rate in a plane normal to the aircraft (A/C) flight-track line. The direction of this flight-track line is commonly referred to as "along track" (ALT), while the scan direction is called "across track" (ACT). Opto-mechanical ACT scanning typically is provided by a Kennedy-type scanner (U.S. Pat. No. 3,211,046, Oct. 12, 1965). The detector elements of the linear array are individually connected one-to-one to a corresponding number of recording channels, either for electronic recording (as in FIG. 2) or for film recording (as in FIG. 5).
Since the A/C velocity (V.sub.A/C) provides the IRLS scanning motion of the array in the ALT direction on the ground and recording is done in a rectilinear format (X-Y Cartesian coordinate system) with a fixed number of recording channels during a given ACT scan, a constant-width swath of ground imagery must be recorded during each scan for contiguous imagery recording in which successive scans are properly spliced together along the entire length of each scan. The width of the swath recorded during each scan period (T.sub.scan) must equal the product: V.sub.A/C.T.sub.scan, which is the ALT distance advanced by the A/C in each scan period. Furthermore, since the number of recording channels and the recorded swath-width must be constant during a given scan, the recorded imagery must have a constant ALT ground scale and "resolution" during a given scan. Further, the ACT scale must be maintained equal to the ALT scale to maintain aspect ratios of the ground imagery. These requirements have been satisfied in prior art IRLS systems by restricting operation to higher altitudes and limiting scan angles to approximately 30 degrees from nadir.
Maintaining the recorded swath-width precisely equal to the distance advanced per scan is a fundamental IRLS requirement if contiguous imagery in successive scans is to be attained. This requirement has previously been met by different combinations of techniques, such as varying the optical scan rate, varying the number of channels recorded, varying the film speed used with a CRT film writer/recorder and switching to additional detector arrays of differing sizes to compensate for A/C altitude and velocity variations. The present invention avoids the inevitable disadvantages in cost, complexity and performance limitations associated with these known techniques while providing continuously-optimized performance.
Even with this complexity of controls, known prior art IRLS systems must operate within a very limited scan angle for a substantially constant range during a given scan, to prevent "bow tie" image distortions (shown in FIG. 3B) due to a fixed array image being swept out over the ground with ever-larger "foot prints" as range, R, increases with wide-angle scanning. The "bow tie" distortion is occasioned by the failure to maintain the recorded swath-width constant, equal to the distance advanced by the aircraft per scan. Thus, it is evident that prior art IRLS systems are inherently incapable of providing wide-angle operation without serious image distortions.
However, a real need exists for a very wide-angle IRLS scanning capability. For example, very wide-angle coverage is a fundamental requirement for adequate ground coverage in tactical reconnaissance missions in which survival necessitates the extremely low-altitude A/C operation now permitted by automatic terrain-following A/C flight control.
Prior art IRLS design must be improved with some form of extremely fast optical zoom or magnification control to meet these new requirements for very wide-angle operation. The zoom or magnification must be continuously and precisely controlled to be proportional to range during the ACT scanning motion, with range varying typically by a factor of at least one hundred to one. Instantaneous range must be computed from A/C altitude above ground level (AGL): H and scan angle from nadir: A. With scanning rates in the order of hundreds of scans per second and with requirements for fast (f/1), high-resolution diffraction-limited optics, use of any known opto-mechanical zoom design is obviously not feasible.
Optical scanning systems which compensate for range variation with scan angle only in the ALT axis are described in applicant's earlier U.S. Pat. No. 4,782,228, assigned to the assignee of the present invention. These systems provide anamorphic zoom, viz., in the ALT direction only. They must be used with prior techniques to compensate for A/C altitude and velocity variations, and must be used with the ACT electronic-zoom technique of the present invention to avoid serious aspect-ratio distortions.